An adhesive sheet comprising an adhesive layer on one side of a resin film is covered with a release liner and stored. Generally, a release liner which is composed of paper or an drawn resin film such as drawn PET or OPP has a shrinkage rate of from about 0 to 0.2% when it is stored at ambient temperature for more than three months. Meanwhile, a resin film for an adhesive sheet is formed in a heat rolling or extruding process and sometimes shrinks by about 1% or more under the same storage conditions as mentioned above. As a result, after storage, wrinkles or curls appear on the adhesive sheet, or the edge of release liner runs out of the edge of the adhesive sheet since the dimension of the release liner does not change but that of the adhesive sheet does. These problematic phenomena deteriorate the appearance of the adhesive sheet.
Another problem is that when an adhesive sheet is applied on an article, air tends to remain between the adhesive layer of the adhesive sheet and the article, which also deteriorates the appearance of the adhesive sheet or impairs adhesion to an article. To resolve these problems, the following literature Nos. 1-6 disclose methods wherein an adhesive sheet is provided with small projections or concave-convex structure on a surface of the adhesive layer, or with continuous grooves as shown in FIG. 1 to form paths for air to flow toward the side surfaces of the adhesive layer.
However, the aforesaid deterioration of appearance caused by shrinkage of the resin film, which occurs when the adhesive sheet is stored together with a release liner thereon, still remains unsolved. Further, when the adhesive sheet with grooves on its adhesive layer as described above is applied to an article, the adhesive agent flows toward the grooves, so that the shape of the grooves on the adhesive layer changes. This change in the shape of the grooves and the flow of the adhesive agent near the grooves affect the appearance of the resin film which is located on the side opposite to the grooves of the adhesive layer, so that a crater-like pattern appears on the surface of the adhesive sheet as if the adhesive sheet is embossed, which significantly deteriorates the appearance of the adhesive sheet after applied.
Meanwhile, paper or a drawn resin film is used as a substrate for a release liner for an adhesive sheet.
For instance, in known release liners with a paper substrate, one or both sides of the paper is treated with a releasing agent such as silicone, or a polyolefin resin is laminated to one or both sides of the paper and the surface of the resin is treated with a releasing agent such as silicone. Paper surface tends to be unlevel. Therefore this is laminated with a polyolefin resin in order to make the unlevelness of the surface of paper less noticeable. Suppression of the concavity and convexity of the paper surface improves surface smoothness of a release liner and, accordingly, smoothness of an adhesive sheet. However, even when a polyolefin resin is laminated with paper, it is difficult to completely eliminate the concavity and convexity of paper. Accordingly, when a release liner with a paper substrate is applied to an adhesive sheet, the concavity and convexity on the release liner surface are transferred to the adhesive layer of the adhesive sheet, so that the adhesive sheet has a drawback that concavo-convex pattern appears on the surface of the adhesive sheet opposite to the side which is in contact with the release liner, when the adhesive sheet is applied to an article. Particularly, in a case of high gloss adhesive sheet which has a mirror-like surface with high gloss, such concavo-convex pattern on the surface deteriorate the mirror-like property and, therefore, such a release liner is unsuitable.
Also known is a release liner, where a surface of a polyolefin resin laminated to paper is treated with a releasing agent and further provided with embossing finish. This liner is suitable for use in adhesive sheets having grooves on the surface of the adhesive layer, but unsuitable for such a high gloss adhesive sheet that has a mirror-like surface with high gloss because concavo-convex pattern caused by paper still remains on the liner surface even when embossing finish is provided.
A release liner where a drawn resin film, such as a drawn PET or OPP, is used as a substrate is also known. A drawn resin film is good in smoothness and, therefore, even when it is applied to an adhesive sheet that has a mirror-like surface with high gloss, smoothness of the adhesive sheet is not impaired. However, the above-mentioned drawn resin film is less suitable for embossing and, therefore, may not be embossed. For instance, a drawn PET has a high melting point of 258 degrees C. and, therefore, it is difficult to supply enough heat to PET to soften it for embossing with a heat roll or the like. If OPP is heated around a melting point, the heating causes significant shrinkage and, therefore, it may not be used as a release liner any more.
Also known is a release film where a release layer of polyolefin is laminated to a polyester film (see, for instance, Patent Literature 7). This release film may have concavo-convex surface caused by deposit of fine particles which are added to improve a sliding property and is not provided with embossing finish. The polyester film is from 5 to 100 micrometers thick.    1. Japanese Utility Model Registration No. 2503717,    2. Japanese Utility Model Registration No. 2587198,    3. Published Japanese Translation of PCT Patent Application No. 9-504325,    4. Japanese Patent Application Laid-Open No. 11-209704,    5. Published Japanese Translation of PCT Patent Application No. 2002-544364,    6. Korean Patent No.253460, and    7. Japanese Patent Application Laid-Open No. 10-114036.
In the drawings, the numerals and the alphabetic symbols mean the following:    1: Adhesive sheet    2: Resin film    3: Adhesive layer    4: Groove    5: Groove    6: Substrate film    A: Polyolefin resin layer    B: Polyolefin resin layer